For style invalidation and style recalc, we always start at the document node traversing the tree. For large, and particularly wide trees, this causes unnecessary traversal of a bunch of elements if we have style recalcs far down the tree only. That's the typical case for :hover, for instance.

We mark the ancestor chain with child-needs-recalc/invalidation, but traversing down the tree, we don't know which child, so we end up traversing all siblings on every level. It's possible to improve on this by storing one or more common ancestors for elements which need an invalidation/recalc.

Simple experimentation showed an improvement from 2ms to 0.1ms hovering the table of contents links in the single-page whatwg html spec. That did not set up the bloom filters and the style sharing depth properly, so numbers are not final. Another test that was done was the PerformanceTests/CSS/ClassInvalidation.html micro benchmark, which showed 5%-10% improvement (only using start node for recalc, not invalidation).